The present invention relates to an optical memory utilizing an injection laser and particularly to such a memory in which the injection laser is not only used as the source of light beam but also as the detector for reading the memory element and for maintaining the proper focus of the light beam on the memory element.
Memory devices have been developed in which the information is recorded and read out optically. One such device includes a rotatable disk having the information recorded thereon as the memory element a radiation source, an optical system for collecting radiation from the radiation source and focusing the radiation onto the disk, and a radiation sensitive detection system for converting into an electrical signal the radiation reflected back from the disk and modulated by the information on the disk. The device also generally includes an additional radiation sensitive detection system for maintaining the focal plane of the optical system at the reflective surface of the disk and thereby maintain the proper focus of the radiation on the disk.
If the radiation source is a semiconductor injection laser, it has been found that the injection laser can also be used as the detection system for the modulated reflected radiation. As described in the article of Y. Mitsuhashi et al. entitled "SELF-COUPLED OPTICAL PICKUP," Optic Communications, Vol. 17, No. 1, Apr. 1976, pp. 95-97, when the modulated radiation is reflected back into the injection laser, certain characteristics of the injection laser such as the light output at constant current, the electrical current through the laser at constant voltages and the voltage drop across the laser at constant current, will vary corresponding to the modulation of the light. Thus, the variation in these characteristics of the injection laser with the modulation of the reflected radiation can be used to indicate the modulation. Therefore, the injection laser can be used as both the source of the radiation and the detector of the modulated radiation reflected from the disk. However, this system still requires a separate detector for maintaining the focal plane of the optical system on the reflective surfaces of the disk.